Frequency-variable oscillator circuit



w. VAN B. ROBERTS t l. 2,408,684 FREQUENCY-VARIABLE OSCILLATOR CIRCUIT Oct. i, 1946. l

Filed Feb. 4, 1945 Patented Oct. l, 1946 FREQUENCY-VARIABLE OSCILLATOR CIRCUIT 'Walter'van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America,

of Delaware a corporation Application February 4, 1943, Serial No. 474,646

(Cl. Z50-36) 1 'ClainL 1 My present invention rrelates lto frequencyvariable oscillator circuits,y and more especially to an voscillatorcircuit adapted to be shifted in frequency without amplitude change. v

One of the main objects of my present invention is to provide a single tube oscillator which may be frequency modulated without amplitude modulation of the oscillatory output energy.

Amore specific object ofV this invention is to provide an oscillator circuit adapted to provide constant-amplitude oscillations, and the frequencyn'of the oscillator circuit Vbeing determined by a :control potential applied to a control grid of the oscillator tube; the latter having separate phase shifter networks in separate connections from the anode and screen electrodes to voltage points on the .tank circuit which are of opposite phase to that of a feedback grid", and the phase shifter networks providing opposite senses of frequency shift.

. AStill other objects of my invention are to provide a single tubey oscillator capable .of frequency deviation free of amplitude change, and `adapted for use in a frequency modulated oscillator ksystem or in an automatic frequency control (AFC) circuit of al superheterodyne receiver.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, -as to both its organization and.

method of operation will best be understoodfby reference to the following description, takenvin connection with the drawing,v in which I have indicated diagrammatically several' circuit organizations whereby ried intoeifect. In' the drawing:v Fig; 1 shows'a circuit employing the invention, Figs. 2a and 2by show vector relations existing in the oscillator circuit,

Fig. 3 schematically illustrates.theapplication o'f. the invention to an AFC system.

Referring, now, tothe accompanying drawing, 'wherein like reference characters in the different .figures designate Similar'circuit elements, there is shown in Fig. 1 an oscillator circuitwhich may r Vbe employed for any desired frequency band,

either in the kilocycle (kc.) or megacycle' (mc.) ranges. The oscillator tube is of the pentode type. Of course, the oscillator is not limited to `anys'peciiic type of electron discharge tube. `It is sucient for the purposes of the present invention that the tube I include a cathode or electron emission electrode 2, a first control grid 3, apositive screen electrode 4, a third controI my invention may be car.

electrode 5 and an anode, or plate; E. It will be understood that other electrodes could be utilized between the cathode 2 and plate 6 to accomplish additional functions, where desired. In the present case cathode 2 is established at an invariable alternating potential, such as ground. The grid 3 is returned to ground through a radio frequency choke coil I in series with a resistor 8. f

.'Ihe Screen electrode/4 is returned to ground through a coil '9 and the` direct current source I0. In other Words, the Screen electrode 4 is connected, through coil 9, to the positiveterminal -of the direct currentsource I il, `and the negative terminal of the latter is grounded. The upper end of coil'9 is `connected through condenser II Ato a desired point on the'coil `I2 of the oscillatory tank circuit I3. The condenser I 4, which is arrangedin shunt with coil I2, tunes the coil I2 to' a desired operating oscillatory frequency. The circuit 'I3 is the usual oscillator tank'circuit,

The plate IiV isconnected to a desired point on coil I2, which may be at a higher or a lower alternating potential than the point. on coil I2 to which electrode d is connected bythe coily I5. The plate end of coil I5 is connected to ground through 'thecondenser IG. The coil 9 and condenser II are shown enclosed in a dotted square which is indicated as Phase shifter. Similarly, coil I5 and condenser I 6 are shown enclosed in a dotted rectangle also -labelled Phase shifter. The phase shifters P1 and P2 are shown terminating at the coil I2 in an adjustable manner vto indicate thatv they may be -adjistably connected to the tank coil.

Ihe control ,grid 5 is shown connected to the negative terminal-of a direct Vcurrent source 20, and the positive terminal of the latter may be connected to av source of frequency control voltage` 'Y This source of frequency control voltage may be either alternating current or direct current. In-'the case of a frequency modulation transmitter, the sourceV of frequency control voltage would be an audio frequency modulation source. However, in the case of AFC of a superheterodyne receivengthe source of frequency control voltage would be the direct Vcurrent voltage output of the usual discriminator-rectifier network. f y y v The plate E is maintained at a positive potenytial relative to ground by means of a direct curgrid 3 is connected to the low potential side of tank circuit I3 through a direct current blocking condenser 22, and coil 23 is illustrated as the output link to the subsequent utilizing circuit.

It Will be realized that Fig. 1 shows a Hartley type oscillator circuit, wherein grid 3 functions as the oscillator grid. The oscillator tube has two output electrodes, viz., the positive screen electrode i and the positive plate 6. The screen 4 and plate 5 are connected to the tank circuit at different points on the other side of the ground tap from the grid 3. As the grid 3 varies in potential, both the screen current and plate current vary in the same sense. The plate current is indicated by the reference character I1. Seen that the voltages introduced by the plate and screen currents respectively in the tank circuit I3 both assist in maintaining oscillation. The screen and plate currents the tank circuit preferably at such points that the voltages induced by the two currents are equal. However, the current I1 passes through the phase shifter Pi. This slightly retards its phase. The screen current I2 passes through the second phase shifter P2, thereby slightly advancing its phase. Accordingly, the voltages induced in the tank circuit by these two currents may be represented .vectorially as in Fig. 2a.

Referring to Fig. 2a, the vectors e1 and e2 correspond to the voltages induced in the tank circuit I3 by the plate current and screen current respectively. The sum of these two vectors, which is the equivalent total voltage in the tank circuit, is shown by the dotted arrow designated e1 plus e2. This total voltage is preferably 180 out of phase with the voltage on grid 3 when the frequency is at its mean value, i. e. in the absence of modulating voltage.

So far, no mention has been made of the control grid 5. The action of this grid is to control the distribution of the total space current of tube I between the screen and plate electrodes 4 and 6 respectively. If the plate voltage is chosen sufficiently low, it is known that a negative potential on grid 5 will cause more and more of the total space current to flow in the screen circuit and less and less in the plate circuit. Although the total space current itself is not affected by grid 5, Fig. 2b shows the result of making grid 5 less negative than normal. In this case the plate current is increased above normal, while the screen current is reduced below normal. Accordingly, the resultant of the two voltages e1 and e2 induced in the tank circuit is of a slightly different phase than in the normal case illustrated in Fig. 2a), It will be noted that the magnitude of the resultant voltage in the case of Fig. 2b is substantially unaltered so that the amplitude of oscillations is not changed.

However, it is well known that a phase shift in any part of the feedback mechanism of an oscillator circuit will produce a frequency change. Hence, under the conditions shown in Fig. 2b, the frequencies will be different from the frequencies prevailing in the normal, or mean, condition depicted in Fig. 2a.. Conversely, making the grid 5 more negative than normal will cause an opposite phase shift in the resultant feedback to the tank circuit and opposite deviation of the oscillator frequency from normal.

It is further pointed out that the relative sup-- ply voltages for the screen Il and plate 6 are preferably so chosen, together with the tapping points of the separate connections from the screen and plate on the tank circuit, that in the absence f rt win be are introduced into Y frequency control voltage substantially equal voltages are induced in the tank circuit I3 by the screen and plate currents I2 and I1. In this way a frequency modulated oscillator is provided whose frequency is caused to deviate both above and below the frequency prevailing in the absence of frequency control voltage.

Where the present invention is employed in a transmitter of frequency modulated carrier wave energy, then electrode of Fig. 1 will be connected to a source of audio modulation energy, and there will be produced across tank circuit I3 frequency modulated oscillations having a mean frequency of a predetermined value. The frequency deviation will depend/upon the relations between the voltages e1 and e2, and will be dependent upon the requirements of the system. It is to be understood that the present invention may also be employed in connection withphase modulated energy, and the generic term angle modulated is to be understood as applying to either of frequency or phase modulated carrier wave energy.

In Fig. 3, I have shown schematically a superheterodyne receiver wherein the present invention is utilized for AFC purposes. Since those skilled in the art are fully acquainted with the constructional details of a superheterodyne receiver, it is not necessary to show such details. Generally, received modulated carrier wave energy is applied to a converter, or rst detector, designated by numeral 3D. The output of the latter, which is at an intermediate frequency (I. F.) is fed to an I. F. amplifier 3l. A second detector 32 demodulates the amplified I. F. energy. The local oscillator, which feeds its energy to the converter circuit, is provided by the tube I. The circuits of the tube I are precisely the same as shown in Fig. 1. The difference resides in the fact that the tank circuit I3 is tuned to a freq-uency which differs from the frequency of the modulated carrier applied to the converter input circuit by a frequency value equal to the operating I. F. value.

The grid 5 has applied to it the AFC voltage output of the discriminator-rectifier network 40. The discriminator-rectier is schematically represented, since those skilled in the art are fully acquainted with the circuits thereof. It is fed with I. F. energy, and the network functions to provide a direct current voltage whose polarity and magnitude are functions respectively of the direction and extent of frequency deviation of the I. F. energy relative to the predetermined operating I. F. value. The AFC voltage output of network 4o is applied through the negative biasing battery 20 to the control grid 5. The discriminator-rectier 40 may be constructed, for example, in the manner disclosed and claimed by S. W. Seeley in his U. S. Patent No. 2,121,103, granted Jun@ 21, 1938. The AFC circuit functions to change the bias of grid 5 in such a manner that the frequency of tank circuit I3 will be shifted to an extent such asto maintain the I. F. energy output of converter 30 close to the predetermined I. F. Value. It is not believed necessary to describe the functioning of the invention in connection with this utilization thereof. Those skilled in the art will fully appreciate the advantages of an AFC system wherein the oscillator circuit does not utilize an auxiliary reactance tube system across the tank circuit, and wherein the oscillator frequency may be directly shifted without appreciable change in the oscillation amplitude. f

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claim.

What I claim is:

In an oscillator circuit comprising an electron discharge tube having at least a cathode, an anode 10 and at; least three successive grids in the space current flowing from the cathode to the anode, a

coil and condenser connected in parallel to provide a tank circuit tuned to a predetermined oscillatory frequency, a source of direct current 15 voltage, means connecting a predetermined intermediate point on said coil to a positive potential point of said source, a high frequency connection from the rst grid adjacent said cathode to 6 one end of said coil, a direct current connection from the anode to a point on the coil which is located on the opposite side of said predetermined intermediate point, a connection from the second grid to a point on the coil on the same side of Asaid intermediate point as the anode point, re-

WALTER VAN B. ROBERTS. 

